1. Field of the Invention
The present invention relates to color image reproduction apparatus and associated methods. More particularly, the invention relates to an imaging system that produces hardcopy images from scenes captured by an electronic camera, and which provides signal processing to improve the color rendition and image quality of the hardcopy images.
2. Description of the Related Art
The two fundamental methods of producing color images are the subtractive method and the additive method. In the subtractive method, image dyes are used to modulate, by absorption, the transmitted or reflected light of the viewing illuminant. In the silver-halide-based photographic system, for example, cyan, magenta and yellow image dyes are used to modulate red, green, and blue light, respectively. In the additive method, modulated intensities of (typically red, green, and blue) lights, known as primaries, are combined to form the color image. One example of a system using the additive method is color television. In this system, the image is typically displayed on a cathode ray tube (CRT). The image on the CRT is produced by modulating the intensities of red, green and blue light-emitting phosphors. The phosphors are therefore the primaries of the system. The signals which modulate the CRT are derived from a set of three linear color image-bearing signals which are typically produced in a television camera using tube or charge coupled device (CCD) sensors. The linear camera signals vary in linear proportion to the sensor exposure levels. For example, if the sensor exposure doubles, the level of the linear signal doubles.
Additive color theory shows that a set of ideal camera spectral sensitivities, called color-matching functions, can be determined based on the primaries of the system. If a camera could be produced with these spectral sensitivities, no further color correction would be required. It can be shown, however, that color-matching functions of any set of realizable primaries will contain negative values, corresponding to areas of negative spectral sensitivity. In practice, then, some electronic signal processing is used to improve color reproduction. The color correction appropriate for such systems is called additive-type color correction. Typically, the linear red, green and blue image-bearing signals in the color camera are linearly combined to form modified red, green, and blue image-bearing signals in such a way as to improve the color reproduction of the CRT display. Both additive color theory and the type of additive color correction used in television systems are discussed in detail in such texts as Principles Of Color Television, John Wiley and Sons, Inc., 1956, incorporated by reference herein.
In the subtractive color photographic system, the displayed photographic image is produced by cyan, magneta, and yellow image dyes. The amounts of the image dyes in the displayed image are based on the exposures originally recorded, as a latent image, by the light-sensitive emulsions of the camera film. In existing subtractive systems, the primaries can be referred to as "unstable", in that changes in the dye concentration of a single dye result in changes in chromaticity. Therefore, unlike the additive system, unique primaries for such systems cannot be determined and thus no set of corresponding color-matching functions can be associated with the set of image dyes. As a result, the color correction appropriate for this type of system is complex. In typical photographic systems, color correction is provided by interlayer interimage effects which adjust the formation of cyan, magenta, and yellow dyes. Due to the nature of the photographic system, these interlayer interimage effects produce adjustments that are non-linearly related to the exposures recorded in the latent image. This type of color correction, which we will refer to as subtractive-type correction, is discussed in detail in such texts as The Theory Of The Photographic Process, MacMillian Publishing Co., Inc. 1977 incorporated by reference herein.
Color imaging systems have been developed which utilize both additive and subtractive elements. For example, telecine systems have been proposed to convert color film images into color television signals. Telecine systems are discussed in The Roll Of Film In The Film-Plus-Telecine System: Considerations About Telecine Design, SMPTE Journal, Vol. 87, September 1978. The appropriate color processing for a telecine would include subtractive-type processing, to correct for the characteristics of the film, followed by additive-type processing, to correct for the characteristics of the television system.
A fundamentally different type of color imaging system which combines both additive and subtractive elements is electronic still photography. In such a system, images are captured using electronic cameras similar to television cameras and viewed on a video display, however, the resulting signals can be used to produce hardcopy images. Presently available electronic photography systems use the still video floppy recording standard and produce "video-quality" images. In this type of system, the output is optimized for presentation on the video display and the video optimized signals are applied directly (without modification) to the hard copy device. The video quality images may be acceptable for today's video display, but prints from still video floppy systems are poor in image resolution and color rendition compared to prints from consumer 35 mm cameras.
To produce a higher quality electronic still photography system, both the resolution and the color reproduction of the hardcopy images must be improved. This invention describes unique signal processing which improves the color reproduction by using a novel method of subtractive-type color processing which attempts to stabilize the primaries associated with the image dyes used to produce the hard copy images, preceded by additive-type processing which attempts to correct the camera sensitivities appropriately for the stabilized primaries. The signal processing further improves the image quality of the hard copy images by performing edge enhancement in parallel with the color correction operations in such a way as to minimize the introduction of undesirable noise and artifacts.